The liquid crystal display apparatus of a transmissive type radiates illumination light on a back face of a liquid crystal display element (hereinafter “liquid crystal panel”) by means of an illuminating apparatus (hereinafter “backlight”) and modulates the transmittance of illumination light by means of the liquid crystal display element and changes brightness to display an image. Although some projectors use reflective liquid crystal display elements, the only difference from the transmissive type is that transmittance changes to reflectivity.
A liquid crystal display apparatus generally emits light on the entire screen at a maximum brightness, irrespective of the display content of an input video signal. However, the proportion of backlight power consumption occupies in the power consumption of the entire liquid crystal display apparatus is significant, and a significant amount of power is consumed wastefully. Further, the liquid crystal panel cannot completely block light, and therefore black cannot be fully reproduced and a display image has lower contrast due to impure black.
Although there is a technique of dynamically changing the light emission brightness of a backlight to solve these problems, if there is a little bright part on a screen to display a video signal, the light emission brightness of backlight cannot be decreased, and therefore it is not possible to provide a great effect. Hence, there is a method (hereinafter “local dimming”) of dividing the screen into a plurality of light emitting areas (about two to several hundreds) and changing light emission brightness in these light emitting areas according to display content matching each light emitting area in the input video signal (see, for example, Patent Literature 1).
According to local dimming, as long as there is no bright pixel in a specific light emitting area, even if there are bright pixels in other light emitting areas, the light emission brightness of backlight for this specific light emitting area can be decreased, so that it is possible to reduce power effectively. Further, the light emitting area belonging to a dark area in an image can keep the light emission brightness of backlight lower, so that it is possible to provide an effect of suppressing impure black of pixels in this light emitting area and enabling display of high contrast.
However, this local dimming generally has a problem that changing the light emission brightness of backlight per light emitting area causes change of brightness in a display image. A technique is proposed to solve this problem by predicting the magnitude of change of brightness and correcting image data (see, for example, Patent Literatures 1 and 2).
With the above technique, it is necessary to accurately learn the light emission brightness of backlight per pixel included in a video signal. Illumination light emitted by the backlight is diffused in a wide range, and therefore accurately predicting the light emission brightness of backlight per pixel not only requires a significant amount of computation but also actually has a great difficulty in terms of an algorithm and is not easy to carry out. However, theoretically, this technique is feasible, if cost and labor are spent.
Hereinafter, an operation of correcting the transmittance of each pixel of a video signal when the light emission brightness of backlight can be accurately predicted per pixel will be described. When the light emission brightness of backlight can be estimated per pixel, it is possible to calculate the corrected transmittance according to equation 1.(Corrected transmittance)=(transmittance)*(maximum light amount)/(estimated light emission brightness value of backlight in pixel of interest)  (Equation 1)
By the way, the light emission brightness of backlight diffuses to lower brightness areas, and therefore the back face light amount in pixels at ends of high brightness areas decreases. Originally, with pixels of high brightness areas, the transmittance is expected to be set at a value close to the maximum value. In this case, the corrected transmittance is set to 1 or more according to the above equation. However, it is not physically possible to set the transmittance to 1 or more, and therefore the transmittance actually needs to be limited to a predetermined maximum transmittance. Therefore, the brightness decreases in pixels at ends of a high brightness area, thereby causing image deterioration such as unevenness in brightness, a decrease of contrast and halo (shade of a low brightness object produced in high brightness areas around a low brightness object).
Similarly, with pixels at ends of black areas, the light emission brightness of backlight in adjacent light emitting areas diffuses, and therefore the back face light amount of the pixels increases. However, the minimum transmittance has a limit, and it is not physically possible to further decrease and correct transmittance, and therefore the transmittance actually needs to be limited to the minimum transmittance. As a result, the brightness of pixels at ends of black areas increases, thereby causing image deterioration such as unevenness in brightness, impure black, a decrease of contrast and halo (blur of a high brightness object produced in black areas around a high brightness object).
The above problem caused by physical device characteristics is difficult to solve no matter how accurately the light emission brightness of backlight is predicted.